Astable multivibrator not producing a square output wave, using two BJTs

Question

I've been trying to build an astable multivibrator circuit using two BC547B NPN transistors.

I know how to build one using a 555 timer, but I'm having some problems using transistors.

I should've gotten a square wave of 0-5 V with a frequency of 500 Hz and duty cycle 0.5, but according to LTspice, I get a flat 5V output.

Can someone suggest where am I going wrong?

Also, I want the current driven to be of the order of some 3-4 mA.

In that case, are the values I've chosen for the resistors and capacitors OK, or should I make some changes?

PS: I'm working on LTspice. I can work only with 2 NPN BJTs of model BC547B, and a single power supply.

Your rrsistors are in parallel, I think the 10k ones are supposed to provide bias current for the bases. — Oskar Skog, Aug 3, 2021 at 6:48
Should I connect them to the bases of the BJT's, in that case? — Abhinav Tahlani, Aug 3, 2021 at 6:50
But, in that case, that would entirely defeat the purpose of switching the transistors on and off to produce the square wave. — Abhinav Tahlani, Aug 3, 2021 at 6:57
Just to add an interesting link. Yet another BJT multivibrator. — jonk, Aug 3, 2021 at 7:18

Andy aka · Accepted Answer · 2021-08-03 07:36:21Z

6

There are mistakes: -

With no DC bias on the bases you will not get an oscillator.

This is how it should be done: -

However, it's not a great or stable circuit.

Try also this circuit (courtesy of micro-cap): -

answered Aug 3, 2021 at 7:22

Andy aka

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\$\begingroup\$ I've tried to build the circuit as the one you've built according to the pic, but the output voltage is a flat 32.3 mV. \$\endgroup\$
– Abhinav Tahlani
Aug 3, 2021 at 7:53
\$\begingroup\$ OK, so I had done the base biasings wrong, it works fine now. Also, I had a follow-up question in my post regarding the current driven. Can you suggest optimal values for the resistors so that the current driven is some 3-4 mA? \$\endgroup\$
– Abhinav Tahlani
Aug 3, 2021 at 7:58
1

\$\begingroup\$ What current driven where? If you mean collector current then look at the waveform in my answer - it gets down to about 0.1 or 0.2 volts and that leaves 4.8 or 4.9 volts across the 100 ohm collector resistors. Ohm's law then tells us that the current through the 100 ohms is about 48 to 49 mA @AmpliFire \$\endgroup\$
– Andy aka
Aug 3, 2021 at 8:59
\$\begingroup\$ Yeah, thanks. I had to scale down the capacitors and scale the resistors up though, to get my desired frequency. Nevertheless, thanks a lot! \$\endgroup\$
– Abhinav Tahlani
Aug 3, 2021 at 16:05

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